Later versions had a built-in BitTorrent client too. It let you not only spoof the user agent, but it let you disable images, disable JS, block content, and a bunch of other settings per site.

It showed a loading progress bar indicating how much of the page content had loaded.

It had an option to only show images that were already cached - useful on very slow connections and better than just turning off all images.

It had mouse gestures for going back/forward, opening new tabs, etc. Oh yeah, it was the first browser to ever implement tabbed browsing.

They had an experiment where you could run decentralized services directly within the browser, called Opera Unite: https://www.howtogeek.com/3468/turn-your-computer-into-a-file-music-and-web-server-with-opera-unite/. They were trying to bring the web back to its original form, where everyone hosted their own content.

All of this was built-in, and yet it was somehow lighter (in terms of RAM usage) than other browsers?

They were truly innovating. We just don't see a lot of software doing that any more. So many companies these days are trying to figure out how to extract more of your personal data and show you more ads.